This invention relates to an aluminum master alloys containing strontium and boron that are used to grain refine and modify the microstructure of Al alloys. More specifically, the invention relates to aluminum-strontium-boron ("Al-Sr-B") and aluminum-strontium-silicon-boron ("Al-Sr-Si-B") master alloys. The introduction of Sr and B into single master alloys provides products capable of accomplishing both grain refinement and morphological modification. Additionally, the combination of B and Sr results in enhanced ductility of the master alloys. The enhanced ductility eases processing of the master alloys into continuous rod products. This invention is especially useful in the grain refinement of hypoeutectic Al-Si alloys.
It is desirable amongst producers and manufacturers of Al alloys to grain refine and modify hypoeutectic Al-Si alloys in order to enhance the physical and mechanical properties thereof. In an unmodified hypoeutectic Al-Si alloy, the silicon-rich eutectic phase has a plate-like morphology such as that shown in FIGS. 1(a) and 1(b). This type of plate-like morphology has a negative affect on the physical and mechanical properties of the alloy. This deleterious affect may be minimized by modifying the structural morphology such that the eutectic phase forms fibers or particles as opposed to plates.
It is known in the art that Sr is an effective modifier for modifying the silicon-rich eutectic phase occurring in Al-Si alloys. See U.S. Pat. No. 4,108,646, U.S. Pat. No. 3,446,170, and K. Alker et al., "Experiences with the Permanent Modification of Al-Si Casting Alloys," Aluminum, 4B(S), 362-367 (1972), each of which is incorporated herein by reference. Typically, the silicon-rich eutectic phase in Al-Si alloys may be modified with an addition of 0.001 to 0.050 weight percent of Sr. Microstructurally, the addition of Sr modifies the microstructure of the eutectic phase thereby precluding formation of the lamellar or platelike structure typically encountered in unmodified alloys, as shown in by FIGS. 1(a) and 1(b). Microstructural modification is especially useful in hypoeutectic Al-Si alloys which enjoy broad commercial application.
Normally, Sr is introduced into the hypoeutectic Al-Si alloy through the addition of a Sr-containing master alloys, such as Al-Sr and Al-Sr-Si. From a practical standpoint, it is desirable that the master alloy contain a significant concentration of Sr in order to minimize the amount of master alloy added to the production alloy to accomplish effective modification. Thus, as the level of Sr increases in the master alloy, the amount of master alloy addition required to attain the desired residual level of Sr in the production alloy decreases, as does the time required to achieve Sr dissolution. Shorter dissolution time equates to shorter holding time in the furnace and reduced energy consumption per heat of finished production alloy. Additionally, shorter holding times lead to higher Sr recovery in the finished heat of production Al-Si alloy. Ultimately, higher Sr levels in the master alloy will result in increased operating efficiency and decreased processing costs for each heat of hypoeutectic alloy treated with such a master alloy. However, as discussed in greater detail below, the use of higher levels of Sr severely limits the degree of workability of the master alloy.
Besides structural modification, it is also desirable to grain refine Al alloys to preclude formation of columnar or twin columnar grains during solidification. It is known in the art that residual Ti, or other transition elements, on the order of 0.001 to 0.20 weight percent, assists in grain refining these alloys. See G. W. Boone et al., "Performance Characteristics of Metallurgical Grain Refiners in Hypoeutectic Al-Si Alloys," in Production, Refining, Fabrication and Recycling of Light Metals, 19:258-263 (1990); and G. K. Sigworth et al., "Grain Refining of Hypoeutectic Al-Si Alloys," AFS Transactions, 93:907-912 (1985), each of which is incorporated herein by reference. Nonetheless, even in the presence of residual Ti, casting conditions can occur whereby the resulting grain structure is too coarse. Thus, in certain instances it is necessary to introduce more effective additives, in addition to Ti, in order to achieve the desired degree of grain refinement.
It has been reported in the literature that an Al-B master alloy provides an excellent grain refining effect for aluminum alloys, so long as the B present in the master alloy is in the form of AlB.sub.2, and not AlB.sub.12, which forms above about 1700.degree. F. See Sigworth et al., "Grain Refining of Hypoeutectic Al-Si Alloys," AFS Transactions, Vol. 93 (1985) p. 907-912, incorporated herein by reference
There are significant problems associated with using a two step inoculation process, i.e., separate additions of B to grain refine and Sr to modify a bath of Al-Si hypoeutectic alloys. The introduction of B as an alloy of Al containing 4-5% B as AlB.sub.2 or AlB.sub.12 and B in solution is usually accompanied by sludging. Typically, the B master alloy is added to the bath while it is still in the furnace (as opposed to the ladle or tundish). Sludging occurs when borides combine with Ti and other transition elements to form intermetallic compounds such as (Al, Ti, V)B.sub.2 which have a specific gravity greater than that of the still molten Al-Si alloy.
When the Sr and B are introduced separately into the bath, the inoculation process requires more time, which means the molten bath must be held in the furnace for a longer period. The result is that the boride particles tend to settle out, thereby forming a "sludge" in the lower or bottom portion of the bath. With infrequent stirring or cleaning, this sludge may tend to agglomerate. It results from long holding times in the furnace after the B addition has been made. This sludging effect can be offset by later additions or by stirring or agitating the bath thereby minimizing agglomeration of boride DKY, WOS, BTD, RDM particles. Nonetheless, a single step inoculation process could eliminate the need for agitation by reducing the holding time of the inoculated hypoeutectic Al-Si bath in the case where modification occurs rapidly following the addition of Sr.
Generally speaking, modifiers and grain refiners are produced in a variety of forms with each form specifically suited for a particular type of finished alloy melting process. Thus, conventional master alloys are available in the form of waffle, ingot, powder, rod, wire, loose chunk, and the like.
In many operations, special feed drive mechanisms have been developed to feed a continuous strand or rod of the master alloy into a molten bath of the alloy being treated. Typically, the continuous rod product is produced in various diameters, including, without limitation, 3/8" rod. The rod is wound about a carrier spool which is mounted directly on or in the vicinity of the feed drive mechanism which feeds the rod-shaped additive into the molten bath. Rod products are produced by rolling, drawing, or extruding bar stock having the desired master alloy composition.
A major advantage to using rod-type products for inoculation of Al-Si hypoeutectic alloys is the elimination of process steps, i.e., weighing the master alloy prior to adding it to the bath. Instead, the rod feeder automatically adds the required length of rod per unit time.
In the case where a short incubation time suffices, an additional benefit of the rod feeder is that it allows a more efficient addition to be made because the master alloy can be added outside the holding or melting furnace. For instance, the inoculation can be made in the tapping trough which transports the molten Al-Si alloy from the furnace to the casting station. The inoculation can then be conducted at lower temperatures, and in less time than would be required for furnace inoculation. The end result is higher recovery of B and Sr in the treated alloy and thus more effective grain refinement and modification in the case where a short incubation time allows this approach to be followed.
As stated earlier, because less volume of master alloy is required, it is desirable to have a master alloy containing a high concentration of Sr, preferably in excess of five weight percent Sr. However, higher levels of Sr severely limits the degree of workability of the master alloy for purposes of producing a rod-type product, so much so that the alloy cannot be successfully continuously rolled.
Specifically, when the Sr content exceeds the solid solubility limit of Sr in Al, an extremely hard, brittle, and semi-continuous intermetallic compound is formed. The intermetallic compound is SrAl.sub.4, which is usually detrimental in master alloys containing Sr in excess of five weight percent. The coarse SrAl.sub.4 that is formed severely limits the ductility, and hence workability, of the master alloy, thereby dictating the final form of the master alloy and the methods by which the master alloy may be manufactured. Consequently, master alloys containing about ten percent Sr up to now have experienced considerable difficulty during continuous rolling, i.e., breakage due to tensile fracture.
Thus, in order to successfully produce a usable, highly alloyed Al-Sr rod product, manufacturers are confined to extruding techniques, which typically do not produce tensile stresses, during fabrication, in order to produce an acceptable rod product. These manufacturing processes, by their nature, are less cost-effective than continuous casting and rolling.
There are a number of practical limitations associated with the extrusion process which results in higher processing costs to the manufacturer and to the end user. Typically, the extrusion process commences by casting a billet of the master alloy, which is then cut to length and placed into the extrusion press whereupon it is subject to hydrostatic compressive loading. The extrusive process forces the bar stock through a die cavity having the diameter of the resultant rod product. As the rod comes out of the extrusion die, it must be wound and packaged onto spools for subsequent use in mechanically driven feeders. Often times, several billets may be required to complete a single spool of rod product. That means that at the end of each billet, the operator must interrupt the extrusion process to remove residual fragments of the remaining billet and insert a new billet in order to add rod to the spool. This interruption in the extrusion process leads to several extrusion defects, including a very rough surface along the initial length of rod until the rod attains critical speed as it exits the die. Preferably this is discarded.
Upon restarting the press with a fresh billet, it may take up to twenty feet or more of initial rod stock through the die in order to attain the critical speed which produces a smooth surface. The rough surface defect is apparent and readily visible to the end user. This defect causes the rod to be brittle and, if excessive, may cause the end user's feed drive mechanism to malfunction due to slippage of the rod product during furnace additions. This sort of malfunction will directly result in a reduced Sr level, below the calculated value, in the finished cast product and may lead to insufficient modification and consequently defective or scrap material.
Additionally, the use of static or semi-continuous casting techniques to form the initial master alloy billet often times introduces excessive oxide particles into the structure of the melt. These particles become entrained in the billet during solidification. Since Sr is a more active oxidizing agent than is Al, a significant portion of the oxide particles formed during casting will be Sr-oxide. It is believed that Sr-oxide does not contribute to the modification of the Al-Si eutectic phase even though the Sr associated therewith is still quantitatively present in the master alloy. Thus, once Sr-oxide is formed in the master alloy, it will not contribute to modification of the treated Al-Sr alloy. Also, the presence of Sr-oxide in the master alloy will result in artificially high recovery levels of Sr. The Sr-oxide effectively precludes or blocks availability of a portion of the Sr being added to the Al-Si alloy from modifying the eutectic phase. Moreover, once these Sr-oxide particles have been introduced into the Al-Si alloy during inoculation, they will be carried into the final product, which can result in reduced fracture toughness, lower tensile strength, and reduced fatigue resistance in the finished product.
Another defect common to extrusion processing is a blister defect which results from non-parallel billet cuts, cold laps, or undersized billets. The blisters result when air is entrapped between the extrusion press housing and the outer surface of the billet.
These types of defects are not present to the same degree on continuously cast and rolled rod stock. Therefore, it would be very advantageous to produce a highly alloyed Sr master alloy which can be continuously cast and rolled.
Thus, there is a significant need for a cost-effective, continuously cast and rolled or conventional form combination master alloy containing about ten percent Sr to provide effective microstructural modification in hypoeutectic Al-Si alloys, along with a second agent that effectively grain refines the treated alloy while further contributing high ductility to the master alloy. These characteristics enhance the processing of the master alloy into a rod product, thereby eliminating the defects commonly encountered in conventionally processed Al-10%Sr master alloy rod products.